Method and apparatus for optical sensing

ABSTRACT

Apparatus and methods for fast quantitative measurement of perturbation of optical fields transmitted, reflected and/or scattered along a length of an optical fiber can be used for point sensors as well as distributed sensors or the combination of both. In particular, this technique can be applied to distributed sensors while extending dramatically the speed and sensitivity to allow the detection of acoustic perturbations anywhere along a length of an optical fiber while achieving fine spatial resolution. Advantages of this technique include a broad range of acoustic sensing and imaging applications. Typical uses are for monitoring oil and gas wells such as for distributed flow metering and/or imaging, seismic imaging, monitoring long cables and pipelines, imaging within large vessel as well as for security applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application, and claims benefit under35 U.S.C. § 120, of U.S. patent application Ser. No. 15/048,315, whichfiled on Feb. 19, 2016. U.S. patent Ser. No. 15/048,315 is acontinuation application and claim benefit under 35 U.S.C. § 120, ofU.S. patent application Ser. No. 13/322,449 (now U.S. Pat. No.9,541,425), filed Nov. 23, 2011, which is a National Stage applicationof International Application No. PCT/GB10/50889, filed May 27, 2010.PCT/GB10/50889 claims priority to GB 0912051.0, filed Jul. 11, 2009 andGB 0908990.5, filed May 27, 2009. These applications are incorporated byreference herein in their entireties.

FIELD OF DISCLOSURE

The present application relates to optical sensors and, in particular,distributed optical fibre sensors and applications thereof.

BACKGROUND OF DISCLOSURE

The benefits of optical fibres have been demonstrated in a number ofsensing applications. The two major areas are: (i) distributed opticalfibre sensors, and (ii) multiplexed point sensor arrays.

Distributed sensors utilise the intensity of backscatter light, withRaman and/or Brillouin peaks in the light signal utilised to measuretemperature, strain or pressure. Distributed sensors offer a number ofadvantages including continuous sensing along the entire length offibre, and flexibility and simplicity of the sensor, which may bestandard telecoms optical fibre. For example, a distributed sensor mayprovide 10,000 measurement points along 10 km of optical fibre with a 1m spatial resolution. Distributed sensor systems therefore offer lowinstallation and ownership costs.

However, due to their slow response, distributed sensors are usuallyonly used in applications where measurements taking in order of severalseconds to hours are acceptable. The most common sensors of this typeare the distributed temperature sensors (DTS), which are made by anumber of companies. A typical performance of a DTS is 1 m spatialresolution and 1° C. temperature resolution in 60 seconds over a 10 kmrange.

Distributed sensors have also been used to measure strain by utilisingBrillouin shifts in reflected or backscattered light, as described inU.S. Pat. No. 6,555,807 [1] or WO 98/27406 [2]. The frequency of theBrillouin shift is about 1 MHz/10με and its linewidth is about 30 MHz.The strain in an order of 10με can be determined along an optical fibreusing the narrow frequency scanning methods described. However, usingthese approaches, the scanning rate is much slower than the pulserepetition rate and measurement times are typically in the order of fewseconds to few minutes.

More recently, a technique for faster measurement of Brillouin frequencyshift has been proposed in U.S. Pat. No. 7,355,163 [3]. This techniqueuses a frequency to amplitude convertor which may be in a form of anoptical fibre Mach-Zehnder interferometer with a 3×3 coupler at itsoutput. However, the strain resolution is limited by the linewidth ofthe Brillouin light and therefore the optical path length difference inthe interferometer should be kept within the coherence length of theBrillouin light. Also, the polarisation fading between the two paths ofthe interferometer, the offset and gain variations of the photodetectorreceivers would significantly limit the strain measurement. Measurementtimes of around 0.1 seconds (10 Hz) with strain resolution of 50με havebeen recently reported using this technique.

For many applications, such as acoustic sensing, much highersensitivities and faster a measurement time in the order of 1millisecond (1 kHz), 0.1 millisecond (10 kHz) or 0.01 millisecond (100kHz) is required.

Multiplexed point sensors offer fast measurements with high sensitivityand are used, for example, in hydrophone arrays. The main applicationfor these in the energy market is for towed and seafloor seismic arrays.However, unlike with distributed sensors, multiplexed point sensorscannot be used where full coverage is required. The size and theposition of the sensing elements are fixed and the number of sensorsmultiplexed on a single fibre is typically limited to 50 to 100elements. Furthermore, the sensor design relies on additional opticalfibre components leading to bulky and expensive array architectures.There is also considerable effort to increase the number of sensors thatcan be efficiently multiplexed on a single length of fibre.

Optical-time-domain reflectometry (OTDR) is a well known technique thathas been used to test optical fibre communications cables. In order toreduce the effect of coherent backscatter interference, which issometime is referred to as Coherent Rayleigh Noise, a broadband lightsource is normally used. However, proposals have also been made in U.S.Pat. No. 5,194,847 [4] to use coherent OTDR for sensing intrusion bydetecting the fast changes in a coherent backscatter Rayleigh signal. Inaddition, Shatalin et al. [5] describes using coherent Rayleigh as adistributed optical fibre alarm sensor.

WO 2008/056143 [6] describes a disturbance sensor similar to that ofU.S. Pat. No. 5,194,847 [4] using a semiconductor distributed feedbacklaser source. A fibre Bragg grating filter of preferably 7.5 GHz is usedto reject out-of-band chirped light and, thereby, improve the coherenceof the laser pulse sent into the fibre. However, this requires matchingof the laser wavelength with the narrow band optical filter, whichresults in the signal visibility variation being reduced compared to asystem which uses a very high coherent source as proposed by U.S. Pat.No. 5,194,847.

Similar techniques have also been proposed for the detection of buriedoptical fibre telecommunication cables (for example in WO 2004/102840[7]), in perimeter security (GB 2445364 [8] and US2009/0114386 [9]) anddownhole vibration monitoring (WO 2009/056855 [10]). However, theresponse of these coherent Rayleigh backscatter systems has been limitedby a number of parameters such as polarisation and signal fadingphenomena; the random variation of the backscatter light; and non-linearcoherent Rayleigh response. Therefore these techniques are mainly usedfor event detection and do not provide quantitative measurements, suchas the measurement of acoustic amplitude, frequency and phase over awide range of frequency and dynamic range.

SUMMARY OF DISCLOSURE

The present disclosure provides novel apparatus and methods for fastquantitative measurement of perturbation of optical fields transmitted,reflected and or scattered along a length of an optical fibre.

The present disclosure can be used for distributed sensors, pointsensors, or the combination of both.

In particular this technique can be applied to distributed sensors whileextending dramatically the speed and sensitivity to allow the detectionof acoustic perturbations anywhere along a length of an optical fibrewhile achieving fine spatial resolution. The present disclosure offersunique advantages in a broad range of acoustic sensing and imagingapplications. Typical uses are for monitoring oil and gas wells, forapplications such as for distributed flow metering and/or imaging;seismic imaging, monitoring long cables and pipelines; acoustic imaginginside large vessels as well as security applications.

The present disclosure may provide apparatus for highly sensitive andfast quantitative measurement of the phase, frequency and amplitude ofthe light transmitted, reflected or scattered along a length of anoptical fibre.

In the prior art, optical couplers have been used in Michelson orMach-Zehnder interferometer configurations where the polarisationbetween the two arms of the interferometer has to be carefullycontrolled. The novel interferometer in the present disclosure allows anm×m coupler to be utilised using non-reciprocal devices, such as Faradayrotator mirrors and an optical circulator, to provide compensated lightinterference with a given phase shift that can be measured at all portsof the optical coupler and analysed very quickly, such as at severaltens of kilohertz.

Embodiments of the disclosure may be used for multiplexed acoustic pointsensors, distributed sensors or a combination of both. In the case ofdistributed sensors, light pulses are injected into the fibre and thephase modulation of the backscattered light is measured along the fibreat several tens of kilohertz. The fibre can be standardtelecommunication fibre and/or cable. Using the techniques describedherein, the sensing system can thereby detect the acoustic field alongthe fibre to provide a distributed acoustic sensor whereby the lengthsof the sensing elements can be selected by a combination of adjustingthe modulation of the light pulse, the path length in the interferometeras well as the sensing fibre configuration.

The data collected along the fibre are automatically synchronised andthey may be combined to provide coherent field images.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the disclosure and how to put it into practice aredescribed by way of example with reference to the accompanying drawingsin which:—

FIGS. 1, 2, 3, and 4 show schematically novel interferometer apparatusaccording to related embodiments of the present disclosure, comprisingcirculators and multiple fibre couplers with different optical pathsthrough the interferometers, Faraday-rotator mirrors and photodetectors;

FIGS. 5 and 6 show schematically how the interferometers can be cascadedaccording to embodiments of the disclosure in series and/or starconfigurations;

FIG. 7 shows schematically a sensor system that utilises theinterferometer of an embodiment of the disclosure for fast measurementof scattered and reflected light from an optical fibre;

FIG. 8 shows schematically a distributed sensor system that utilises theinterferometer of an embodiment of the disclosure to generate a seriesof pulses each of different frequency and thereby allowing a differentportion of the scattered light to interfere with another portion of thescattered light with a slight frequency shift resulting in a heterodynebeat signal;

FIG. 9 is a block diagram representing a data processing methodaccording to an embodiment of the disclosure;

FIG. 10 is a block diagram representing a method of calibrating theinterferometer according to an embodiments of the disclosure;

FIG. 11 shows schematically a distributed sensor system the spectrum ofthe light that is modulated using a fast optical modulator, thatgenerators multiple frequency side bands with part of spectrum beingselected using an optical filter.

FIG. 12A shows the spectrum of the light modulated and selected usingthe optical filter for the arrangement shown in FIG. 11;

FIG. 12B shows schematically a tinning diagram for a method inaccordance with FIG. 11;

FIG. 13 shows schematically an embodiment in which the fibre can bedeployed as linear sensors, directional sensors or in a multidimensionalarray of sensors;

FIGS. 14 to 16 show schematically alternative arrangements of an opticalfibre for use in embodiments of the disclosure;

FIGS. 17 to 18 schematically show applications of the disclosure invarious aspects.

DETAILED DESCRIPTION

FIG. 1 shows a first embodiment, generally depicted at 100, of a novelinterferometer for measuring the optical amplitude, phase and frequencyof an optical signal. The incoming light from a light source (not shown)is preferably amplified in an optical amplifier 101, and transmitted tothe optical filter 102. The filter 102 filters the out of band AmplifiedSpontaneous Emission noise (ASE) of the amplifier 101. The light thenenters into an optical circulator 103 which is connected to a 3×3optical coupler 104. A portion of the light is directed to thephotodetector 112 to monitor the light intensity of the input light. Theother portions of light are directed along first and second opticalpaths 105 and 106, with a path length difference between the two paths.Faraday-rotator mirrors (FRMs) 107 and 108 reflect the light backthrough the first and second paths 105 and 106, respectively. TheFaraday rotator mirrors provide self-polarisation compensation alongoptical paths 105 and 106 such that the two portions of lightefficiently interfere at each of the 3×3 coupler 104 ports. The opticalcoupler 104 introduces relative phase shifts of 0 degrees, +120 degreesand −120 degrees to the interference signal, such that first, second andthird interference signal components are produced, each at a differentrelative phase.

First and second interference signal components are directed by theoptical coupler 104 to photodetectors 113 and 114, which measure theintensity of the respective interference signal components.

The circulator 103 provides an efficient path for the input light andthe returning (third) interference signal component through the sameport of the coupler 104. The interference signal component incident onthe optical circulator 103 is directed towards photodetector 115 tomeasure the intensity of the interference signal component.

The outputs of the photodetectors 113, 114 and 115 are combined tomeasure the relative phase of the incoming light, as described in moredetail below with reference to FIGS. 7 and 9.

Optionally, frequency shifters 110 and 111 and/or optical modulator 109may be used along the paths 105 and 106 for heterodyne signalprocessing. In addition, the frequency shift of 110 and 111 may bealternated from f1, f2 to f2, f1 respectively to reduce anyfrequency-dependent effect between the two portions of the lightpropagating through optical paths 105 and 106.

The above-described embodiment provides a novel apparatus suitable forfast quantitative measurement of perturbation of optical fields, and inparticular can be used for distributed and multiplexed sensors with highsensitivity and fast response times to meet requirements of applicationssuch as acoustic sensing.

FIG. 7 shows an application of the interferometer of FIG. 1 to thedistributed sensing of an optical signal from an optical system 700. Itwill be apparent that although the application is described in thecontext of distributed sensing, it could also be used for point sensing,for example by receiving reflected light from one or more point sensorscoupled to the optical fibre.

In this embodiment 700, light emitted by a laser 701 is modulated by apulse signal 702. An optical amplifier 705 is used to boost the pulsedlaser light, and this is followed by a band-pass filter 706 to filterout the ASE noise of the amplifier. The optical signal is then sent toan optical circulator 707. An additional optical filter 708 may be usedat one port of the circulator 707. The light is sent to sensing fibre712, which is for example a single mode fibre or a multimode fibredeployed in an environment in which acoustic perturbations are desiredto be monitored. A length of the fibre may be isolated and used as areference section 710, for example in a “quiet” location. The referencesection 710 may be formed between reflectors or a combination of beamsplitters and reflectors 709 and 711.

The reflected and the backscattered light generated along the sensingfibre 712 is directed through the circulator 707 and into theinterferometer 713. The detailed operation of the interferometer 713 isdescribed earlier with reference to FIG. 1. In this case, the light isconverted to electrical signals using fast low-noise photodetectors 112,113, 114 and 115. The electrical signals are digitised and then therelative optical phase modulation along the reference fibre 710 and thesensing fibre 712 is computed using a fast processor unit 714 (as willbe described below). The processor unit is time synchronised with thepulse signal 702. The path length difference between path 105 and path106 defines the spatial resolution. The photodetector outputs may bedigitised for multiple samples over a given spatial resolution. Themultiple samples are combined to improve the signal visibility andsensitivity by a weighted averaging algorithm combining thephotodetector outputs.

It may be desirable to change the optical frequency of the lightslightly to improve the sensitivity of the backscattered or reflectedsignals. The optical modulator 703 may be driven by a microwavefrequency of around 10-40 GHz to generate optical carrier modulationsidebands. The optical filter 708 can be used to select the modulationsidebands which are shifted relative to the carrier. By changing themodulation frequency it is possible to rapidly modulate the selectedoptical frequency.

Data Processing

FIG. 9 schematically represents a method 1100 by which the optical phaseangle is determined from the outputs of the photodetectors 113, 114,115. The path length difference between path 105 and path 106 definesthe spatial resolution of the system. The photodetector outputs may bedigitised for multiple samples over a given spatial resolution, i.e. theintensity values are oversampled. The multiple samples are combined toimprove the signal visibility and sensitivity by a weighted averagingalgorithm combining the photo-detector outputs.

The three intensity measurements I₁, I₂, I₃, from the photodetectors113, 114, 115 are combined at step 1102 to calculate the relative phaseand amplitude of the reflected or backscattered light from the sensingfibre. The relative phase is calculated (step 1104) at each samplingpoint, and the method employs oversampling such that more data pointsare available than are needed for the required spatial resolution of thesystem.

Methods for calculating the relative phase and amplitude from threephase shifted components of an interference signal are known from theliterature. For example, Zhiqiang Zhao et al. [12] and U.S. Pat. No.5,946,429 [13] describe techniques for demodulating the outputs of 3×3couplers in continuous wave multiplexing applications. The describedtechniques can be applied to the time series data of the presentembodiment.

For each sampling point, a visibility factor V is calculated at step1106 from the three intensity measurements I₁, I₂, I₃, from thephotodetectors 113, 114, 115, according to equation (1), for each pulse.V=(I ₁ −I ₂)²+(I ₂ −I ₃)²+(I ₃ −I ₁)²  Equation (1)

At a point of low visibility, the intensity values at respective phaseshifts are similar, and therefore the value of V is low. Characterisingthe sampling point according the V allows a weighted average of thephase angle to be determined (step 1108), weighted towards the samplingpoints with good visibility. This methodology improves the quality ofthe phase angle data 1110.

Optionally, the visibility factor V may also be used to adjust (step1112) the timing of the digital sampling of the light for the maximumsignal sensitivity positions. Such embodiments include a digitiser withdynamically varying clock cycles, (which may be referred to herein as“iclock”). The dynamically varying clock may be used to adjust thetiming of the digitised samples at the photodetector outputs for theposition of maximum signal sensitivity and or shifted away frompositions where light signal fading occurs.

The phase angle data is sensitive to acoustic perturbations experiencedby the sensing fibre. As the acoustic wave passes through the opticalfibre, it causes the glass structure to contract and expand. This variesthe optical path length between the backscattered light reflected fromtwo locations in the fibre (i.e. the light propagating down the twopaths in the interferometer), which is measured in the interferometer asa relative phase change. In this way, the optical phase angle data canbe processed at 1114 to measure the acoustic signal at the point atwhich the light is generated.

In one or more embodiments of the present disclosure, the dataprocessing method 1100 is performed utilising a dedicated processor suchas a Field Programmable Gate Array.

Sensor Calibration

For accurate phase measurement, it is important to measure the offsetsignals and the relative gains of the photo-detectors 113,114 and 115.These can be measured and corrected for by method 1200, described withreference to FIG. 10.

Each photodetector has electrical offset of the photodetectors, i.e. thevoltage output of the photodetector when no light is incident on thephotodetector (which may be referred to as a “zero-light level” offset.As a first step (at 1202) switching off the incoming light from theoptical fibre and the optical amplifier 101. When switched off, theoptical amplifier 101 acts as an efficient attenuator, allowing nosignificant light to reach the photodetectors. The outputs of thephotodetectors are measured (step 1204) in this condition to determinethe electrical offset, which forms a base level for the calibration.

The relative gains of the photodetectors can be measured, at step 1208,after switching on the optical amplifier 101 while the input light isswitched off (step 1206). The in-band spontaneous emission (i.e. theAmplified Spontaneous Emission which falls within the band of thebandpass filter 102), which behaves as an incoherent light source, canthen be used to determine normalisation and offset corrections (step1210) to calibrate the combination of the coupling efficiency betweenthe interferometer arms and the trans-impedance gains of thephotodetectors 113, 114 and 115. This signal can also be used to measurethe signal offset, which is caused by the in-band spontaneous emission.

Conveniently, the optical amplifier, which is a component of theinterferometer, is used as in incoherent light source without arequirement for an auxiliary source. The incoherence of the source isnecessary to avoid interference effects at the photodetectors, i.e. thecoherence length of the light should be shorter than the optical pathlength of the interferometer. However, for accurate calibration it ispreferable for the frequency band of the source to be close to, orcentred around, the frequency of light from the light source. Thebandpass filter 102 is therefore selected to filter out light withfrequencies outside of the desired bandwidth from the AmplifiedSpontaneous Emission.

When used in a pulsed system, such as may be used in a distributedsensor, the above-described method can be used between optical pulsesfrom the light source, to effectively calibrate the system during use,before each (or selected) pulses from the light source withsubstantively no interruption to the measurement process.

Variations to the above-described embodiments are within the scope ofthe disclosure, and some alternative embodiments are described below.FIG. 2 shows another embodiment, generally depicted at 200, of a novelinterferometer similar to that shown in FIG. 1 but with an additionalFaraday-rotator mirror 201 instead of photodetector 112. Like componentsare indicated by like reference numerals. In this case the interferencebetween different paths, which may have different path length, can beseparated at the three beat frequencies f₁, f₂ and (f₂−f₁). Thearrangement of this embodiment has the advantage of providing additionalflexibility in operation, for example the different heterodynefrequencies can provide different modes of operation to generatemeasurements at different spatial resolutions.

FIG. 3 shows another embodiment of a novel interferometer, generallydepicted at 300, similar to the arrangement of FIG. 1, with likecomponents indicated by like reference numerals. However, thisembodiment uses a 4×4 coupler 314 and an additional optical path 301,frequency shifter 304, phase modulator 303, Faraday-rotator mirror 302and additional photo-detector 308. In this case the interference betweendifferent paths, which may have different path length differences, canbe separated at the three beat frequencies (f₂−f₁), (f₃−f₂) and (f₃−f₁).Alternatively, the Faraday-rotator mirror 302 may be replaced by anisolator or a fibre matched end so that no light is reflected throughpath 301, so only allowing interference between path 105 and 106.

The 4×4 optical coupler of this arrangement generates four interferencesignal components at relative phase shifts of −90 degrees, 0 degrees, 90degrees, 180 degrees.

FIG. 4 shows another embodiment of the interferometer. In this case anadditional path is introduced in the interferometer by inserting aFaraday-rotator mirror 402 instead of the photo-detector 112.

In all of the above-described embodiments, optical switches may be usedto change and/or select different combinations of optical path lengthsthrough the interferometer. This facilitates switching between differentspatial resolution measurements (corresponding to the selected pathlength differences in the optical path lengths).

FIGS. 5 and 6 show examples of interferometer systems 500, 600 arrangedfor used in cascaded or star configurations to allow the measuring ofthe relative optical phase for different path length differences. InFIG. 5, three interferometers 501, 502, 503 having different path lengthdifferences (and therefore different spatial resolutions) are combinedin series. In FIG. 6, four interferometers 602, 603, 604 and 605 havingdifferent path length differences (and therefore different spatialresolutions) are combined with interferometers 602, 603, 604 inparallel, and interferometers 603 and 605 in series. In FIG. 6, 601 is a3×3 coupler, used to split the light between the interferometers.Arrangement 600 can also be combined with wavelength divisionmultiplexing components to provide parallel outputs for differentoptical wavelengths.

The embodiments described above relate to apparatus and methods for fastquantitative measurement of acoustic perturbations of optical fieldstransmitted, reflected and or scattered along a length of an opticalfibre. The disclosure in its various aspects can be applied orimplemented in other ways, for example to monitor an optical signalgenerated by a laser, and/or to monitor the performance of a heterodynesignal generator, and to generate optical pulses for transmission intoan optical signal. An example is described with reference to FIG. 8.

FIG. 8 shows a system, generally depicted at 800, comprising aninterferometer 801 in accordance with an embodiment of the disclosure,used to generate two optical pulses with one frequency-shifted relativeto the other. The interferometer receives an input pulse from a laser701, via optical circulator 103. A 3×3 optical coupler 104 directs acomponent of the input pulse to a photodetector, and components to thearms of the interferometer. One of the arms includes a frequency shifter110 and an RF signal 805. The interference between the two pulses ismonitored by a demodulator 802. The light reflected by Faraday-rotatormirrors 107 and 108 is combined at the coupler 809 using a delay 803 tomatch the path length of the interferometer, so that the frequencyshifted pulse and the input pulse are superimposed. The coupler 809introduces relative phase shifts to the interference signal, andinterferometer therefore monitors three heterodyne frequency signalcomponents at relative phase shifts. The optical circulator 103 passesthe two pulses into the sensing fibre.

In this embodiment, the reflected and backscattered light is notdetected by an interferometer according to the present disclosure.Rather, the reflected and backscattered light is passed through anoptical amplifier 804 and an optical filter 806 and are then sent to afast, low-noise photodetector 807. The electrical signal is split andthen down-converted to baseband signals by mixing the RF signal 805 atdifferent phase angles, in a manner known in the art. The electricalsignals are digitised and the relative optical phase modulation at eachsection of the fibre is computed by combining the digitised signalsusing a fast processor 808.

FIG. 11 shows another embodiment of apparatus for point as well asdistributed sensors. In this case the modulation frequency 704 of theoptical modulator 703 is switched from f1 to f2 within the optical pulsemodulation envelope.

The optical filter 708 selects two modulation frequency sidebands1202/1203 and 1204/1205 generated by the optical modulator as indicatedin FIG. 12. The frequency shift between first order sidebands 1202 and1203 is proportional to the frequency modulation difference (f2−f1)whereas the frequency shift between 2^(nd) order sidebands 1204 and 1205is proportional to 2(f2−f1). Therefore, the photo-detector output 806generates two beat signals, one of which is centred at (f2−f1) and theother at 2(f2−f1). Using the demodulator 901, the relative optical phaseof the beat signals can be measured independently. The two independentmeasurements can be combined to improve the signal visibility, thesensitivity and the dynamic range along the sensing fibre.

FIG. 12A shows the modulation spectrum of the light and the selection ofthe sidebands referred to above.

FIG. 12B shows the original laser pulse 1206 with pulse width of T atfrequency f₀ which is modulated at frequency f1, f2 and f3 during aperiod T1, T2 and T3, respectively. The delay between T1, T2 and T3 canalso be varied. One or more modulation sidebands is/are selected withthe optical filter 708 to generated a frequency shifted optical pulsesthat are sent into the fibre. The reflected and/or backscatter signals(709, 710, 711 and 712) from the fibre from is directed to aphotodetector receive via a circulator 707. The reflected and orbackscatter light from different pulses mix together at thephotodetector output to generate heterodyne signals such (f2−f1),(f3−f1), (f3−f2), 2(f2−f1), 2(f3−f1) and 2(f3−f2). Other heterodynesignals are also generated but (2f2−f1), (2f3−f1), (2f1−f2), (2f1−f3),(2f3−f1) and (2f3−f2) are also generated at much higher frequencies. Theheterodyne signal are converted down to base band in-phase andquadrature signals. The in-phase and quadrature signals are digitise bya fast analogue to digital convertors and the phase angle is computedusing fast digital signal processor.

FIG. 13 shows an embodiment with distributed sensors with the sensingfibre 702 subjected to different perturbation fields 1302, 1304 and1307. The sensing fibre can be used as linear sensors 1303 and 1304, asdirectional sensors 1305 and 1306 or as multidimensional array sensors1308, 1309 and 1310. Since all the measurements are synchronised, theycan be processed to enhance the signal sensitivity, achieve a widedynamic range and provide field imaging using beam forming techniques.

FIG. 14 shows an optical fibre arrangement 1400, where the fibre isplaced on a surface area in a continuous path without crossing overanother part of the fibre to increase the sensitivity, in a doublefigure-eight pattern.

FIG. 15 shows an optical fibre arrangement 1500, where the fibre isplaced on a surface area in a continuous path without crossing overanother part of the fibre to increase the sensitivity, in a foldedthree-Omegas (Ω Ω Ω) pattern.

These arrangements are particularly useful to increase the sensingsensitivity, frequency response and the spatial resolution of thesensing system, while simplifying installation techniques and minimisingbending losses.

FIG. 16 shows an optical fibre arrangement 1600, where the fibre isplaced in a logarithmic spiral pattern to form an acoustic camera ortelescope. Acoustic energy can be detected all along a section of fibre.In this case the signals detected along the field are synchronised andusing addition signal processing such as beam forming, the near-fieldand far-field acoustic emission can be mapped. Such an apparatus can beused to look far into the sky, through oceans, deep into the ground, orwithin vessels. This aspect also provides apparatus for monitoring theenvironmental noise such as aircraft noise during take-off and landingas well as noise from other flying objects or natural habitats.

FIG. 17 shows at 1700 an application to distributed flow sensing along apipe 1702 at different sections with fibre 1701 wrapped around the pipeat separated locations 1704 and attached or placed close to the pipe viaclamps 1706 to measure the flow noise and pressure variations. Thisarrangement may also be used to monitor the operation of injector orcontrol valves 1708, and sensors may be used for in-well perforatedzones monitoring and sand production monitoring. For example, forin-well applications, the acoustic noise profile can be used to measurethe flow by noise logging at every location along the well. In addition,the noise spectrum can be used to identify the phase of the fluid.Further noise spectrum correlation techniques can be used over a longsection of the well to determine the speed of sound as well as trackingeddies generated within the flow to accurately determine the flow rates,using analysis techniques for example as described in WO 2006/130499[14]. This document describes an array of optical fibre acousticinterferometric sensors used to track the speed of the vortices inducedpressure waves as a function of the flow. However, the interferometersrequire discrete components, such as Bragg grating pairs, and a limitednumber of sensors over a short section of a pipe can be practically beused. With the distributed acoustic sensor of the present disclosure wecan use a flexible method of attaching to or placing close to a pipe acontinuous length of optical in an optimised configuration along entirelength of pipe. For example the spatial resolution measurements may beincreased by wrapping the fibre around the pipe to track the vorticesinduced pressure waves or simply track the acoustic waves generated andpropagated along the pipe to determine the speed of sound both in thesame and opposite directions of the flow. The speed of sound is afunction of the fluid composition and by mapping the speed of sound onecan visualise how the flow profile changes along the pipe.

Also, since we do not require any discrete components, a higheroperating temperature can be achieved with proper coating protectionapplied on to the fibre. The fibre sensitivity can also be enhanced orreduced using different coatings or jackets. Also, the fibre can be madeinto a continuous cable with an enhanced sensing sensitivity whileproving a protection for the fibre in harsh environments.

FIG. 18 shows at 1800 an application to dynamic positioning of a riser1802 using acoustic fibre optic sensors 1804 and acoustic referencesources 1806 whereby the optical fibre sensor 1804 measures the time offlight of acoustic signals received at different locations along theriser and thereby determines the position of the riser.

Review of Features of the Disclosure in Various Aspects and Embodiments

In one aspect, the present disclosure provides an optical interferometerapparatus which can provide multiple path differences between theoptical signals and provide interference signals between differentoptical paths with fixed and/or variable phase shifts. Theinterferometer utilises beam splitting components, circulating devicesand Faraday rotator mirrors in a novel configuration. The opticalsignals at the output of the interferometer are converted to electricalsignals which digitised for fast processing. The offset levels of theelectrical signals are removed and their amplitude are normalised. Therelative phase shifts of optical signals are accurately determined bycombining the normalised electrical signals.

In another aspect, the present disclosure relates to an interferometerapparatus that utilises beam splitters and non-reciprocal devices toprovide light interference with given phase shifts and path lengthdifferences that can be measured at all ports of the beam splitterswhereby the relative phase modulation of the light can be computed veryaccurately and quickly, such as at every few nanoseconds. Theinterferometer may use optical fibre components such as an m×m fusedoptical fibre coupler that is connected to an optical fibre circulatorat one of its ports; Faraday-rotator mirrors that reflect and, at thesame time, provide polarisation compensation for the light propagatingthrough the different paths of the interferometer and photodetectorsthat are used to measure the interference light signals. The incomingoptical light may be amplified using an optical fibre amplifier, andpreferably the interferometer has a pass band optical filter to filterout the out of band Amplified Spontaneous Emission noise (ASE). Theinterferometer may provide birefringence compensation for lightpropagating along different optical paths through the interferometer.This provides sufficiently high visibility at the outputs of theinterferometer.

In another of its aspects, the present disclosure provides a method forcompensating the offset and the gain of the photo-detectors, and thecoupling ratio of the interferometer arms, to normalise the resultantinterference signals used to measure the relative phase of the modulatedinput light in any of preceding claims where the detector offset ismeasured by switching off the optical amplifier in the backscatter path;the resultant photo-detector offset and gain then being determined byswitching on the amplifier while the input light is switched off; theASE of the optical amplifier then acts as an independent incoherentlight source and thereby the offsets and relative gains of thephoto-detectors can be determined and the detected light signalsnormalised. The method may therefore use incoherent light that entersthe input of the interferometer to normalise the relative signalamplitudes at the output of the photo-detectors. For example, when anoptical preamplifier is used at the input of the interferometer, thespontaneous light emission can be used to measure the combination of thesplitting ratio of the interferometer arms and the relative gains of thephoto-detectors and thereby normalise the relative signal amplitudesaccordingly.

Another additional feature of the present disclosure is to use phasemodulators and/or frequency shifters to shift the relative frequency andor vary the phase between the optical paths of the interferometer.Frequency shifters and/or phase modulators may be used to provideheterodyne signals and/or to separate the resultant interference lightsignal from different paths through the interferometer.

An additional feature of an embodiment of the disclosure is selectingthe frequency of the frequency shifter sufficiently high so that atleast one cycle of the beat frequency results within one light pulseresolution. Different frequency shifts may be used between differentoptical paths of the interferometer for the separation and/or heterodynedetection of the phase between different optical paths. The frequencyshifts between different optical paths may be alternated to correct forany frequency dependency of the interferometer output signals.

An additional feature of an embodiment of the disclosure is theselection of different optical paths through the interferometer such asby using optical switches. The optical switches may be used to selectdifferent optical paths through the interferometer and thereby select adifferent spatial resolution measurement. Another aspect of the presentdisclosure relates to a system comprising a number of interferometerscascaded in a series or in a star configuration or a combination ofboth.

The disclosure also provides a system that utilises a light pulse formultiplexed and/or distributed sensors by measuring the phase modulationof the reflected and/or the backscattered light along a length of fibrewith high sensitivity, high dynamic range and a high speed of over tensof kilohertz. In this way, the present disclosure can provide amultiplexed and/or distributed acoustic sensing system.

An additional feature of an embodiment of the disclosure is digitisingthe outputs of the interferometer, or the photodetectors of theinterferometer, at least twice over a spatial resolution interval. Anadditional feature of an embodiment of the disclosure is combining theoutputs of the interferometer to determine the insensitive measurementsample points resulting from any signal fading of the light in order toreject and/or provide a weighted signal average of the multiple samplesof the light over a given spatial resolution measurement or interval.Embodiments of the disclosure use a digitiser with dynamically varyingclock cycles, (which may be referred to herein as “iclock”), to adjustthe timing of the digital sampling of the light for the maximum signalsensitivity positions. The dynamically varying clock may be used toadjust the timing of the digitised samples at the photo-detector outputsfor the position of maximum signal sensitivity and or shifted away wherelight signal fading occurs.

A further aspect of one or more embodiments of the disclosure providesfrequency shifted light, using a fast optical modulator to generatesidebands, preferably with a suppressed carrier spectrum, and aband-pass optical filter to select the modulation sidebands whereby themodulation frequency is varied rapidly between two portions of lightpulse propagating through the optical modulator. The optical modulatormay also chop off a portion of light pulse in the middle so as togenerate two pulses with different frequencies. In this case thereflected and/or the backscattered light generated by the two pulses arecombined to result in a heterodyne signal whose phase is determined tomeasure the relative optical phase modulation along the sensing fibre.

Providing multiple heterodyne signals can improve the dynamic range andreduce the effect of signal fading. When the scattered and/or thereflected light from the two pulses are combined, the modulationsidebands generate different beat frequencies which are proportional tothe modulation frequency difference and to the order of the sidebands.The frequency of the light may be changed to optimise the signalsensitivity over a given section of the fibre. The frequency of thelight passing through the optical modulator may be changed rapidly sothat at least two portions of light pulse have different modulationsideband frequencies and, in addition, part of the light pulse may bechopped to generate two distinct portions of light pulses with differentmodulation sideband frequencies. The modulation sidebands between thetwo portions of the light pulse scattered or reflected from a sensingfibre may beat together to generate multiple heterodyne signals atmultiples of the frequency difference between the two pulses that areproportional to the order of the modulation sidebands.

Embodiments of the disclosure may use a laser light or a broadband lightsource. Coherent matching of the light with the same delay results in aninterference signal that can be used to measure the relative phasemodulation of the scattered or reflected light along the fibre.Embodiments disclosed herein may use wavelength division multiplexedcomponents to utilise multiple laser light pulses with differentwavelengths and, preferably, varying time shift with respect to each tocontrol the cross-phase modulation between the light pulses and to allowthe processing of multiple pulses in the sensing fibre without andcross-sensitivity to allow the system to achieve a higher measurandfrequency response. This may be the acoustic frequency response of thesystem to provide a different spatial sampling resolutions and/orpositions, and/or to allow the efficient rejection of any points withlow sensitivity.

An additional feature of one or more embodiments of the disclosure isthe selection of different spatial resolutions whereby the sensitivityand the frequency response along the sensing fibre can be adjusted, andthe dynamic range can be widened.

The sensing fibre may be standard single mode fibre, polarisationmaintaining fibre, a single polarisation fibre, and or a ribbon fibre,and it can be coated and or cabled to enhance or to suppress itssensitivity.

An additional feature of one or more embodiments of the disclosure isthe selection of different configurations of the fibre to optimise thesensitivity, the frequency and the directionality of the sensing fibreat different locations. The fibre may be deployed as linear sensors,direction sensors or multidimensional array sensors. The fibre may beplaced on a surface area in a continuous path without crossing overanother part of the fibre to increase the sensitivity, the frequencyresponse and or the spatial resolution of the sensor system such as in afolded three-Omegas (Ω Ω Ω) and or double eights (88) configurations.This is particularly useful to increase the sensing sensitivity,frequency response and the spatial resolution of the sensing system,while simplifying installation techniques and minimising bending losses.

The fibre may be attached on a surface of a vessel to listen to thenoise generated within the vessel to monitor the changes in the process,acoustically image the process, as well to detect any leaks.

A further aspect provides an apparatus using acoustic sensors fordistributed flow measurement and imaging, in-well perforated zonesmonitoring and sand production monitoring. For example, for in-wellapplications, the acoustic noise profile can be used to measure the flowby noise logging at every location along the well. In addition, thenoise spectrum can be used to identify the phase of the fluid. Furthernoise spectrum correlation techniques can be used over a long section ofthe well to determine the speed of sound as well as tracking eddiesgenerated within the flow to accurately determine the flow rates.

The sensor systems may be used as a distributed acoustic sensor,enabling the determination of distributed flow measurement and imaging,perforated zones monitoring and sand production monitoring in oil andgas wells and flowlines. The distributed temperature and strainmeasurements may be combined to enhance the data interpretation of thedistributed acoustic sensor.

A further application is listening along previously installed opticalfibres for surveillance applications. This includes measurements alongfibres installed along boreholes, pipelines, perimeters, ports andborders.

An additional aspect provides a dynamic positioning apparatus usingacoustic fibre optic sensors and acoustic reference sources whereby theoptical fibre sensor measures the time of flight of acoustic signalsreceived at different locations along the structure and therebydetermines its position.

A further aspect provides pipeline structure monitoring apparatus usingan acoustic fibre sensor and a pig that emits a sound (known as a“whistling pig”). The optical fibre sensor measures the acoustictransmission through the wall of the pipe for diagnostics as well as fortracking the position of the pig.

Another aspect provides pipeline monitoring apparatus where the sensingfibre is deployed inside the pipeline and carried along the pipeline bythe fluid drag to provide a measurement of the noise flow fordiagnostics of the pipeline as well as for flow characterisation and/orimaging.

Another aspect provides an apparatus using a fibre sensor used foracoustic sensing and an energy harvesting self-powered acoustic sourceto generate sufficient acoustic emission that can be picked up by anearby sensing fibre for data communication, measurement, diagnosticsand surveillance applications including along long pipelines, in-welland in other remote applications.

Another aspect of the present disclosure provides an apparatus usingacoustic fibre sensors to measure seepage rates along dams and dykes bygenerating an acoustic noise source in the upstream reservoir or in thecore of the dam and measuring the acoustic signal strength detectedalong the fibre whereby areas of seepage act as low acoustic impedancepaths for acoustic wave transmission and thereby exhibiting loudersignal levels.

Other advantages and applications of the present disclosure will beapparent to those skilled in the art. Any of the additional or optionalfeatures can be combined together and combined with any of the aspects,as would be apparent to those skilled in the art.

CONCLUDING REMARKS

As has been described above, apparatus and methods for fast quantitativemeasurement of perturbations of optical fields transmitted, reflectedand/or scattered along a length of an optical fibre. In particular, thepresent disclosure can be used for distributed sensing while extendingdramatically the speed and sensitivity to allow the detection ofacoustic perturbations anywhere along a length of an optical fibre whileachieving fine spatial resolution. The present disclosure offers uniqueadvantages in a broad range of acoustic sensing and imagingapplications. Typical uses are for monitoring oil and gas wells such asfor distributed flow metering and/or imaging, monitoring long cables andpipelines, imaging of large vessels as well as security applications.

There follows a set of numbered features describing particularembodiments of the invention. Where a feature refers to another numberedfeature then those features may be considered in combination.

1. An optical sensor system comprising: a light source generating apulsed optical signal; an optical sensing fibre configured to receivethe optical signal; an optical modulator for generating frequencysidebands in the optical signal; an optical filter configured tocontrollably select one or more of the modulation sidebands, and,thereby vary the frequency of the light input to the sensing fibre.

2. The system of feature 1, where the frequency of the light is changedto optimise the signal sensitivity over a given section of the fibre.

3. The system of feature 1 or feature 2, where the frequency of thelight passing through the optical modulator is changed rapidly so thatat least two portions of light pulse have different modulation sidebandfrequencies.

4. The system of any preceding feature wherein part of the light pulseis chopped to generate two distinct portions of light pulses withdifferent modulation sideband frequencies.

5. The system of feature 4 wherein the modulation sidebands between thetwo portions of the light pulse scattered or reflected from a sensingfibre beat together to generate multiple heterodyne signals at multiplesof the frequency difference between the two pulses that are proportionalto the order of the modulation sidebands.

6. The system of any preceding feature wherein the light source is alaser light or a broadband light source.

7. The system of any preceding feature wherein using wavelength divisionmultiplexed components to utilise multiple laser light pulses withdifferent wavelengths and, preferably, varying time shift with respectto each to control the cross-phase modulation between the light pulsesand to allow the processing of multiple pulses in the sensing fibrewithout and cross-sensitivity to allow the system to achieve a highermeasurand frequency response, such as higher acoustic frequencyresponse, and to allow the efficient rejection of any points with lowsensitivity.

8. The system of any of the above features where the sensing fibre is asingle mode fibre, polarisation maintaining fibre, a single polarisationfibre, multimode fibre and or a ribbon fibre.

9. The sensor system of any preceding feature used as a distributedacoustic sensor.

10. The sensor system of feature 9 where the distributed sensor can beconnected to standard optical fibre for pipelines, perimeters, ports orborder security.

REFERENCES

-   [1] U.S. Pat. No. 6,555,807, Clayton et al.-   [2] WO 98/27406, Farhadiroushan et al.-   [3] U.S. Pat. No. 7,355,163, Watley et al.-   [4] U.S. Pat. No. 5,194,847, Taylor et al.-   [5] Shatalin, Sergey et al., “Interferometric optical time-domain    reflectometry for distributed optical-fiber sensing”, Applied    Optics, Vol. 37, No. 24, pp. 5600-5604, 20 Aug. 1998.-   [6] WO 2008/056143, Shatalin et al.-   [7] WO 2004/102840, Russel et al.-   [8] GB 2445364, Strong et al.-   [9] US 2009/0114386, Hartog et al.-   [10] WO 2009/056855, Hartog et al.-   [11] WO 2007/049004, Hill et al.-   [12] Zhiqiang Zhao et al., “Improved Demodulation Scheme for Fiber    Optic Interferometers Using an Asymmetric 3×3 Coupler”, J. Lightwave    Technology, Vol. 13, No. 11, November 1997, pp. 2059-2068-   [13] U.S. Pat. No. 5,946,429, Huang et al-   [14] WO 2006/130499, Gysling et al.

What is claimed is:
 1. A method of monitoring the position of astructure using a distributed acoustic sensor (DAS) system, the method,comprising: deploying a plurality of acoustic reference sources in anarea in which a structure to be monitored is located; deploying anoptical fiber distributed acoustic sensor system on the structure, theoptical fiber distributed acoustic sensor system comprising a sensingoptical fiber that extends along the structure, the optical fiber beingarranged in use to receive a pulsed optical signal, the optical fiberbeing configured to constrain the pulsed optical signal such that thepulsed optical signal propagates along the length of the optical fiberin a first direction, wherein the pulsed optical signal is backscatteredand/or reflected along the length of the optical fiber, thebackscattered and/or reflected light being constrained by the opticalfiber such that the backscattered and/or reflected light propagatestherealong in a second direction opposite the first direction; emittingacoustic signals from the plurality of acoustic reference sources; usingthe optical fiber distributed acoustic sensor system to detect, at aplurality of sections along the length of the sensing optical fiber, theacoustic signals emitted by the plurality of acoustic reference sources,wherein acoustic signals incident on the plurality of sections cause theoptical fiber to expand and contract such that the backscattered and/orreflected light is modulated, and wherein the optical fiber distributedacoustic sensor system is configured to: i) receive the backscatteredand/or reflected light from along the length of the optical fiber; andii) process the received backscattered and/or reflected light to therebydetect the acoustic signals, wherein the processing comprises measuringthe optical phase modulation of the backscattered and/or reflected lightreceived from each contiguous section of optical fiber along the lengthof optical fiber based on the time taken for the pulsed optical signalto propagate along the length of the optical fiber in the firstdirection, and the time taken for the backscattered and/or reflectedlight to propagate back along the optical fiber in the second direction,to thereby map the received backscattered and/or reflected light to arespective section of optical fiber; calculating relative positions ofthe plurality of sections along the optical fiber in dependence on thedetected acoustic signals from the acoustic reference sources; and fromthe calculated positions of the sections along the optical fiber,determining a position the structure.
 2. A method according to claim 1,wherein the using the optical fiber distributed acoustic sensor todetect the acoustic signals emitted by the plurality of acousticreference sources comprises measuring the time of flight of the emittedacoustic signals received at the plurality of sections.
 3. A methodaccording to claim 1, wherein the processing further comprises measuringthe relative phase, frequency and amplitude of the backscattered and/orreflected light received from along the length of the sensing opticalfiber.
 4. A method according to claim 3, wherein the relative phase,frequency and amplitude measurements taken along the length of thesensing optical fiber are synchronised.
 5. A method according to claim1, wherein receiving the backscattered and/or reflected light isperformed using an interferometer, the interferometer comprising atleast two optical paths with a path length difference therebetween suchthat the received backscattered and/or reflected light interferes in theinterferometer to produce interference components.
 6. A method accordingto claim 1, wherein the structure is a subsea riser.
 7. A system formonitoring the position of a structure using a distributed acousticsensor (DAS) system, the system comprising: a plurality of acousticreference sources arranged to emit acoustic signals; and a distributedacoustic sensor (DAS) system, the distributed acoustic sensor systemcomprising a sensing optical fiber deployed such that the fiber extendsalong a structure to be monitored, the optical fiber being arranged inuse to receive a pulsed optical signal, the optical fiber beingconfigured to constrain the pulsed optical signal such that the pulsedoptical signal propagates along the length of the optical fiber in afirst direction, wherein the pulsed optical signal is backscatteredand/or reflected along the length of the optical fiber, thebackscattered and/or reflected light being constrained by the opticalfiber such that the backscattered and/or reflected light propagatestherealong in a second direction opposite the first direction, thedistributed acoustic sensor system being arranged to: i) receive thebackscattered and/or reflected light from along the length of theoptical fiber; ii) process the received backscattered and/or reflectedlight to thereby detect, at a plurality of sections along the length ofthe sensing optical fiber, the acoustic signals emitted by the pluralityof acoustic reference sources, wherein acoustic signals incident on theplurality of sections cause the optical fiber to expand and contractsuch that the backscattered and/or reflected light is modulated, whereinthe processing comprises measuring the optical phase modulation of thebackscattered and/or reflected light received from each contiguoussection of optical fiber along the length of optical fiber based on thetime taken for the pulsed optical signal to propagate along the lengthof the optical fiber in the first direction, and the time taken for thebackscattered and/or reflected light to propagate back along the opticalfiber in the second direction, to thereby map the received backscatteredand/or reflected light to a respective section of optical fiber; iii)calculate relative positions of the plurality of sections along theoptical fiber in dependence on the detected acoustic signals from theacoustic references sources; and iv) from the calculated positions ofthe sections along the fiber, determine a position the structure.
 8. Asystem according to claim 7, wherein the distributed acoustic sensorsystem is arranged to measure the time of flight of the acoustic signalsreceived at the plurality of sections.
 9. A system according to claim 7,wherein the distributed acoustic sensor system further comprises: anoptical source arranged to input the pulsed optical signal to thesensing optical fiber; means for receiving the backscattered and/orreflected light from along the sensing optical fiber; and means forprocessing the received backscattered and/or reflected light to detectthe emitted acoustic signals.
 10. A system according to claim 9, whereinthe means for processing is further arranged to measure the relativephase, frequency and amplitude of the backscattered and/or reflectedlight received from along the length of the sensing optical fiber.
 11. Asystem according to claim 10, wherein means for processing is furtherarranged to synchronise the relative phase, frequency and amplitudemeasurements taken along the length of the sensing optical fiber.
 12. Asystem according to claim 9, wherein the means for receiving is aninterferometer comprising at least two optical paths with a path lengthdifference therebetween such that the received backscattered and/orreflected light interferes in the interferometer to produce interferencecomponents.
 13. A system according to claim 7, wherein the structure isa subsea riser.